The Laboratory Simulation Tests Of The New Vacuum Preloading Methods And Research On Settlement Prediction

The Dredger fill means a kind of man-made deposits which is pumped by dredgerand mud pump from the offshore or channel to the specified position. Coastal citiesusually take dredger fill as the raw material for land building project. However, the specialsoil needs to be processed to use as it has many characteristics of high moisture content,high porosity ratio, high compressibility, high clay content, low strength, low permeabilityand so on. The premise of using vacuum preloading method in engineering is the bearingcapacity of dredger fill, that means the surface of dredger fill would become hard shellcommonly in2or3years. And under the powerful vacuum suction effect, particles willmove with the water to drain pipes, the large plastic pipes are surrounded with large fineclay of dredger fill and then are formed mud membrane, which seriously influence thedrainage consolidation of dredger fill and cause lower consolidation efficiency. So, it isnecessary to looking for a suitable vacuum preloading consolidation method for high highclay content dredger fill. And when take the dredger fill with high clay content and highorganic matter content as building foundation in the process of use, the post-constructionsettlement is often very large, and the time of settlement reaches to stabilization is verylong, often needs to reach for years, or even decades, very adverse to engineering. So it isvery meaningful to conduct the thorough in view of the secondary consolidationdeformation characteristics of dredgrer fill and influence factors to the forecast for thedevelopment of the settlement of dredger fill.According to the above problems, this paper first made through research on the basicproperties of dredger fill from Tianjin and Dalian, discussed the engineering geologicalproperties of high clay content dredger fill. Then used the straight-line vacuum preloadingmethod to reinforce Dalian dredger fill with laboratory simulation test, making sure that pipes type and distance were main purpose in straight-line vacuum preloading process. Inthe process of the laboratory simulation test, monitoring the variations of vacuum degree,settlement and pore water pressure, after the test, tested and analysised the mechanicsproperties, physical and chemical properties, microstructural features, Comparing thereinforcement effect of the six model boxes on different types and distance of drain pipes.The results shown that straight-line vacuum preloading can greatly reduced the vacuumpressure relay frictional cost, improve the strength of the soil and shorten the constructionperiod.The above straight-line vacuum preloading experiment can shorten the period ofconsolidation, but the experiment analysis found that the phenomenon of drainageplugging was still serious. Thus, the paper used the step vacuum preloading method toreinforce Tianjin dredger fill with laboratory simulation test in order to improveconsolidation efficiency of high clay dredger fill. The main concept of this method is to setup proved staged and stepped vacuum degree instead of one-time80kPa vacuum loadwith the consolidation of dredger fill, that is soil water separation stage, self-weightdeposit stage, vacuum preloading20kPa-40kPa-80kPa stage, making the water out of thesoil body gradually, achieving the goal of reducing the clay grain migration and unevensettlement. In the test process, monitoring the settlement, displacement and pore waterpressure. after the test, and after each stage, testing and analysised the mechanicsproperties, physical and chemical properties, microstructural features, comparing thereinforcement effect of dredger fill in different stage.The soil consolidation deformation includes not only the primary consolidationdeformation, but also the secondary consolidation deformation. The primary consolidationdeformation is a deformation process that gradually eliminating the water in the pore ofsoil and the excess pore water pressure dissipating gradually, the effective stress increasinggradually, until the excess pore water pressure is completely dissipated; the secondaryconsolidation deformation is after the excess pore water pressure is completely dissipated,under the condition of effective stress is invariable, the soil particle surface hydrated filmintegrating and soil particle structure adjusting caused as time growth and slow theincrease of soil skeleton creep deformation. For the dredger fill after vacuum preloading,the post-construction settlement is very large, cannot be ignored. Thus, this paper testedand analysised the primary consolidation deformation and secondary consolidation characteristics of Tianjin and Dalian dredger fill, it is proved the primary consolidationcoefficient and secondary consolidation coefficient were not with a constant, but with theconsolidation pressure changes, compared the primary consolidation and secondaryconsolidation properties; Also discusses the influence of the state of soil consolidation,pre-load, add lotus and add lotus time to the secondary consolidation coefficientThe settlement curves of the straight-line vacuum preloading and the step vacuumpreloading were not smooth linear, there were many discontinuous points indwell them,existed some certain errors, that made the settlement curves deviating from the real value.The sources on errors were affected by external environment factors and human readingerrors and the instrument measuring precision, and the degree of influence wasunknowable, That influence made the settlement curves deviating from the real value andaffected on the final settlement prediction, to accurately predict the soil settlement was adifficult problem. According to this problem, the nonlinear wavelet threshold denosingmethod was used to disposal the monitoring settlement curves of Tianjin and Daliandredger fill, extracted stable data to better predict the final settlement.At last, according to the problem that commonly theory calculate methods onlyconsidered the primary consolidation settlement, and ignoring the secondary consolidationsettlement, the prediction results had a certain gap with actual settlement, made thecalculation results difficulty to meet the design requirements. So the BP neural networkmodel was put forward to predict the settlement of dredger fill. And training the settlementdata of the straight-line vacuum preloading and the step vacuum preloading after denoised,fully learning the regularity of samples, predicting the final settlement and analysis theprediction results.This paper was baesd on laboratory simulation test, proposed two vacuum preloadingmethods: the straight-line vacuum preloading and the step vacuum preloading, usedphysical, chemical and microscopic test means, comparatively analysised the materialmigration rules, the strength properties and microstructure characteristic of the twomethods in the process of consolidation; and analysised the primary consolidation andsecondary consolidation characteristics, discussed influencing factors of the secondaryconsolidation coefficient; at last, using the nonlinear wavelet threshold denosing methodto disposal the monitoring settlement curves, eliminated the influence of the accidentalerrors to the settlement curves, and then set up the BP neural network to predict the final settlement, analysised the reinforcement effect of the two methods, provided a theory basisfor speeding up the consolidation efficiency of dredger fill, and finally served forengineering.